Vapor Extraction Research Articles

Analytical Solutions for Steady‐State Gas Flow in Layered Soils with Field Applications

AbstractSoil vapor extraction (SVE) is widely used to remove volatile organic compounds from the vadose zone. Design of SVE systems rely largely upon vacuum responses and limited vapor concentration data measured during short‐term soil gas extraction tests performed in single extraction wells. Interpretation of such vacuum data is often simply a rule of thumb as most field sites have layering complexity negating applicability of existing analytical models. This paper provides the derivation of an analytical model for steady, axisymmetric gas flow in heterogeneous (layered) soils from a single well. A general, variable flow boundary condition along the well screen represents actual conditions more closely than a uniform flow or uniform well pressure condition. Each soil layer is assumed homogeneous with anisotropic gas permeability. The solution is derived using the generalized integral transform technique and includes expressions for vacuum, velocities, and streamlines. The model is applied to the interpretation of multiple well tests at a field site and uses linear superposition to extend the flow model to multi‐well extraction. The demonstration site included an array of vacuum monitoring data collected during nine individual well flow tests. A method of normalizing the vacuum data is illustrated that allowed the full data set to be employed in a single calibration effort. The test site also included a surface cap with an apparent vertical permeability two to three orders of magnitude smaller than the sands of the vadose zone. This large permeability contrast posed no difficulties in evaluating the solution.

Analytical and numerical study of thermal and solvent-based gravity drainage for heavy oil recovery

With the increasing demand for energy, an initial screening for heavy oil recovery options is important. However, the calculation of the gravity drainage recovery of heavy oil is complex and involves coupled fields and changes in the properties of heavy oil or its mixtures. There is currently no general analytical treatment for heavy oil gravity drainage processes, such as steam-assisted gravity drainage (SAGD), vapor extraction (VAPEX) and warm VAPEX, with the simplest possible assumptions but with the necessary mechanisms. In this study, a novel discrete hypothetical model has been proposed to simplify the calculation of gravity drainage flow in porous media. Then, a validated equation with a reasonable mixing rule is used to describe the property changes of heavy oil or its mixtures under decoupled temperature and concentration variables. Finally, internal velocity distributions for different gravity drainage processes are validated with numerical modeling results. Our results agree with previous work on the importance of mass transfer coefficients. Moreover, different from the previous work, the effect of slightly water-soluble solvents (dimethyl ether) has been investigated on diffusion in both the oil phase and the water phase. For solvent-based heavy oil recovery, a slightly water-soluble solvent shows great advantages with promising application potentials. The results of this study can guide the initial screening for a gravity drainage process and the analysis of its key mechanisms.

Transient Simulation of 1, 4-Dioxane Fate and Transport During Enhanced Soil Vapor Extraction Treatment

1, 4-Dioxane is an emerging contaminant a that is fully miscible in water and resistant to biodegradation. The presence of 1, 4-Dioxane in vadose zone creates prolonged environmental issues and risk human health. To date, treatment technologies for 1, 4-Dioxane vadose zone remediation are still being developed. In this work, a numeric simulation tool was developed to investigate the fate and transport of 1, 4-Dioxane in the treatment zone of enhanced soil vapor extraction (XSVE) which utilizes heated air injection to improve contaminant removal efficacy. The results suggested identify source zone is critical for XSVE treatment, inaccurate heat injection point would prolong treatment duration.

Effects of organic carbon content on in situ remediation time using steam–air injection

Abstract Groundwater contamination by chlorinated hydrocarbons (CHC) is a common phenomenon that poses health risks to both humans and animals. These halogenated hydrocarbons infiltrate into the soil matrices and form pools at the bottoms of the aquifers thus contaminating the groundwater sources. Thermally enhanced soil vapour extraction (TSVE) using steam–air injection has gained popularity as an alternative technique to remediate the saturated and vadose source zones contaminated with CHC. This technique has been successfully applied in the remediation of contaminated sites (brownfields, industrial sites) and groundwater. However, the presence of organic carbon (OC) contents within the soil matrices has not been intensively studied. This paper, therefore, intends to contribute toward increasing the understanding of the effects of OC on the remediation time using TSVE. A 2-D flume experimental model was conducted in VEGAS laboratory using coarse sand, fine sand and silty soil with 0, 1 and 2% addition of the activated carbon as OC to investigate the desorption time of PCE and TCE as CHC during TSVE extraction using steam–air injection. 100 kg of soil mixed with the activated carbon was treated with 50 g TCE and 50 g PCE and then remediated using TSVE. The remediation times were recorded and recovered CHC was documented. It was discovered that the presence of OC enhanced the adsorption of the CHC onto the soil matrices thereby increasing the time required for the complete remediation of the contaminant from the soil. An increase of OC by 1% resulted in desorption time by a factor of 4–7.

A novel transcritical-recuperative two-stage Organic Rankine Cycle for dual/multi-source heat recovery applications

The Transcritical Regenerative Series Two-stage Organic Rankine Cycle (TR-STORC) has improved performance over STORC and single-stage ORC. However, the TR design requires vapor extraction between turbine stages, which prevents the use of compact two-stage ORC turboexpanders. An alternative Transcritical-Recuperative (TREC) design is proposed which eliminates vapor extraction allowing direct integration of compact two-stage turbines. The novel TREC-STORC operates with a recuperator acting as an additional evaporator at the low-pressure turbine exit (LP mode). Another TREC design requiring vapor extraction from the high-pressure turbine stage (HP mode), similar to the TR design, is also presented for comparison purposes. TREC-STORC in LP mode presents a marginally improved performance of 1–3% over HP mode for various IC engine heat source conditions. With cyclopentane as working fluid, at engine design point conditions, LP mode delivers 4% higher power outputs over HP mode and TR-STORC. Compared to STORC and single-stage ORC, the LP mode of TREC-STORC delivers 21% and 28% higher power outputs, respectively. For other working fluids, TREC-STORC (LP mode) delivers between 2–5% additional power output over TR-STORC, with 18–38% and 20–56% increased power outputs over STORC and single-stage ORC, respectively. TREC-STORC (LP mode) presents a superior and robust ORC architecture for dual/multi-source heat recovery applications.

Systematical study on dimethyl ether as a renewable solvent for warm VAPEX and its significant implications for the heavy oil industry

As part of its Healthy Environment and Healthy Economy (HEHE) plan to address climate change, the Canadian government intends to increase the carbon tax from its current level of $30 per tonne to $170 per tonne over the next nine years. To respond to this plan positively, oil companies have started to address the most significant issue on carbon emissions from their current oil recovery processes, such as steam assisted gravity drainage (SAGD), by utilizing more environmentally responsive processes. Vapor extraction (VAPEX) was proposed as an alternative method; however, its previous studies were limited to a conventional solvent (n-alkane), which is not economically feasible. Dimethyl ether (DME), an emerging renewable chemical resource, was initially proposed as a solution to reduce carbon emissions and aroused interest in the heavy oil industry. But limited research has been carried out for understanding and evaluating DME’s performance in heavy oil recovery.In this study, essential mechanisms including asphaltene deposition, reservoir responses due to solvent injection or solid adsorption, and mass transfer by considering water phase solubility during DME injection have been studied by numerical simulation. Moreover, production performance, in-situ upgraded oil and energy efficiency have been compared among SAGD, conventional propane VAPEX and DME as a novel solvent for warm VAPEX. DME has been evaluated as a promising solvent for heavy oil and bitumen recovery with a less negative blockage effect, a higher recovery factor and rate, higher API upgrading, and higher energy efficiency compared with a conventional solvent. DME, as a renewable, amphoteric chemical solvent, will lead to significant implications in addressing the challenges brought by carbon emissions.

Combined Application of Ozone and Hydrogen Peroxide to Degrade Diesel Contaminants in Soil and Groundwater

Environmental pollution has been a major concern in recent times, and soil and groundwater pollution are areas which have received particular focus. This has led to the development of various remediation techniques such as excavation, soil vapor extraction, bioremediation, chemical oxidation, and so on. Among all remediation techniques, chemical oxidation has been proven to be the most effective and feasible technique around the world. In this study, various combinations of ozone and hydrogen peroxide were used to treat diesel-contaminated soil and groundwater in an experimental setup. Experimental soil and groundwater were prepared with properties similar to the contaminated soil. An ozone generator and a pump injection system were deployed for combining ozone and hydrogen peroxide. Five different experiment batches were prepared based on the hydrogen peroxide concentration and its ratio to the soil. The diesel concentration in the water dropped from 300 mg/L to 7 mg/L in the first hour of treatment, which dropped below the detection limit (0.01 mg/L) thereafter. Similarly, 63.9% degradation was achieved with the combined sparging of ozone and hydrogen peroxide in the soil. Ozone combined with 7% hydrogen peroxide was the most promising combination for removing the contaminants. In addition, this research explored the hydroxyl radical conversion rate of ozone and the perozone, the difference in order of magnitude is greater than one which shows that the perozone has better oxidation capacity than ozone only. The findings of this study show that combining ozone with hydrogen peroxide is a competent and feasible onsite remediation method for diesel contaminants in soil and groundwater. Thus, this method can be applied in local gas stations, accidental spillage sites, and small-scale refineries for onsite treatment in a cost-effective and technically sound way within a short time span.

Transient migration behavior of VOC vapor in layered unsaturated soils subjected to multiple time-dependent point pollution sources: Analytical study

Predicting the migration behavior of volatile organic compounds (VOCs) vapor is essential for the remediation of subsurface contamination such as soil vapor extraction. Previous analytical prediction models of VOCs migration are mostly limited to constant-concentration nonpoint sources in homogeneous soil. Thus, this study presents a novel analytical model for two-dimensional transport of VOCs vapor subjected to multiple time-dependent point sources involving transient diffusion, sorption and degradation in layered unsaturated soils. Two representative time-dependent sources, i.e., an instantaneous source and a finite pulse source, are used to describe the short-term and long-term leakage. Results reveal that soil heterogeneity can cause pollution accumulation, especially in low-diffusivity capillary fringe. The assumption of an equivalent plane source from multiple point sources would significantly overestimate the vapor concentration and the contaminated range. The previous single point source model is no longer inapplicable when the relative distance and/or the release interval between sources is small, giving a strong interaction between multiple sources. Moreover, a faster vapor degradation rate or a higher groundwater level will reduce the area of vapor plume linearly. Hence, close attention should be paid to the time-variation characteristics of multiple sources, the vapor degradation and the groundwater level fluctuation in practice to facilitate soil remediation. The proposed model is a promising tool for addressing the above issue.

Determination of individual concentration-dependent diffusivity of binary gas-mixtures in reservoir-fluid systems

Diffusion is one of the main mass transfer principles associated with a gas injection process, e.g., vapor extraction (VAPEX) and cyclic solvent injection (CSI), to reduce the viscosity of heavy oil by dissolution of light gas(es). A few efforts have been made to estimate the diffusivity of each gas component of a binary gas-mixture in heavy oil as a constant, but no attempts have been made to determine individual diffusivity of each gas component in the binary gas-mixture as concentration-dependent. In this study, the concentration-dependent diffusivities of CO2 and C3H8 in a CO2/C3H8 mixture, which preferentially diffuses in a Lloydminster heavy oil, are evaluated implicitly as a power function of gas concentration, pressure, and temperature through the oil viscosity. The coefficient and exponent of such a power function for each gas are determined once the deviations between the measured and calculated values for oil swelling factors and composition of each gas dissolved in heavy oil at the completion of a dynamic volume analysis (DVA) test are minimized by applying the 1D time-dependent finite element method (FEM). Compared with the constant diffusivities for the same test, considering concentration-dependency of the dissolved gases reproduces the experimentally measured oil swelling factors and compositions at the end of the test with a significantly enhanced accuracy. Then, such obtained concentration-dependent diffusivities are extended to reproduce the experimentally measured swelling factors of a pressure-decay test performed with another CO2/C3H8 gas-mixture and the same heavy oil. From such an evaluation, it can be inferred that the coefficient and exponent of the diffusivity correlation associated with a binary gas-mixture are functions of pressure, temperature, feed gas composition, and test duration, respectively. This is because the coefficient and exponent for one gas component may be affected by the presence of the other as the latter alters the composition of heavy oil in which the former diffuses.

Experimental investigation of innovative superheated vapor extraction technique in heavy oil reservoirs: A two-dimensional visual analysis

Steam-assisted gravity drainage (SAGD) has been widely employed for heavy oil exploration, which however suffers low oil displacement efficiency and high-water consumption. Conventional vapor extraction (VAPEX) remediates those issues, nevertheless, accompanied by low oil production rate with cold or warm solvents injection. Here, an innovative thermal VAPEX was investigated for when injecting superheated solvents (>160OC). The thermal properties of several solvents were compared to indicate the impacts of temperature and solvent concentration. Three sets of two-dimensional visualization experiments were conducted to elucidate the changes of temperature field and solvent chamber, characterized by oil recovery, cumulative oil exchange rate and solvent retention. The results showed that a hot solvent with a higher carbon number resulted in a larger latent heat of vaporization and a better viscosity reduction effect by dissolution and heating. The viscosity reduction rate of n-pentane (below 10 wt%) was approximately 70%. Solvent chamber developed step-wisely: rising stage, lateral expansion stage and descending stage. Over liquid solvent, solvent vapor rendered greater sweep efficiency (75%), displacement efficiency (85%), much lower solvent retention (10%), high oil recovery (65%), and cumulative oil exchange rate (8 t/t). Current studies provided a reliable experimental basis to further exploit thermal vapor on site.

Foam flushing with soil vapor extraction for enhanced treatment of diesel contaminated soils in a one-dimensional column

A limitation of soil vapor extraction (SVE) remediation for total petroleum hydrocarbons (TPHs) in the unsaturated zone is the inability to remove the less volatile petroleum mixture compounds in diesel fuel. SVE combined with foam flushing may have the potential to enhance dissolution or mobilization of soil sorbed diesel and allow mobilized diesel to move to the SVE extraction well. A nonionic surfactant polyoxyethylene (20) sorbitan monooleate (TW80) was selected for generating foam, and a procedure to incorporate the oxidant sodium persulfate (SPS) in generating TW80/SPS foam to deliver chemical oxidation, was also studied. Both TW80 and TW80/SPS foams exhibited 96–98% quality under 8–32 mM of TW80 and 10–50 mM of SPS. The addition of SPS in TW80 solution resulted in elevated ionic content and degradation of TW80, which may reduce the foam stability and have minor effects on foam quality. Through analysis of interrelationships among column flushing experimental parameters, it was shown that the foam quality was reduced to 42–47% when foam flushed through a diesel contaminated soil column. Moreover, the results of column flushing tests operated for 12 h indicated that the effectiveness of removal of diesel by different foams followed the order of TW80 foam (53%) > TW80/SPS foam (37%) >N2 gas flow alone (3%). It was shown that foam flushing could be an alternative approach, rather than using N2 gas flow alone (SVE), in enhancing SVE for reducing diesel contamination in the unsaturated zone.

A Rapid Decision Support Tool for Estimating Impacts of a Vadose Zone Volatile Organic Compound Source on Groundwater and Soil Gas

AbstractDiminishing rates of subsurface volatile contaminant removal by soil vapor extraction (SVE) oftentimes warrants an in‐depth performance assessment to guide remedy decision‐making processes. Such a performance assessment must include quantitative approaches to better understand the impact of remaining vadose zone contamination on soil gas and groundwater concentrations. The spreadsheet‐based Soil Vapor Extraction Endstate Tool (SVEET) software functionality has recently been expanded to facilitate quantitative performance assessments. The updated version, referred to as SVEET2, includes expansion of the input parameter ranges for describing a site (site geometry, source characteristics, etc.), an expanded list of contaminants, and incorporation of elements of the Vapor Intrusion Estimation Tool for Unsaturated‐zone Sources software to provide soil gas concentration estimates for use in vapor intrusion evaluation. As part of the update, SVEET2 was used to estimate the impact of a tetrachloroethene (PCE) vadose zone source on groundwater concentrations, comparing SVEET2 results to field‐observed values at an undisclosed site where SVE was recently terminated. PCE concentrations from three separate monitoring wells were estimated by SVEET2 to be within the range of 6.0–6.7 μg/L, as compared to actual field concentrations that ranged from 3 to 11 μg/L PCE. These data demonstrate that SVEET2 can rapidly provide representative quantitative estimates of impacts from a vadose zone contaminant source at field sites. In the context of the SVE performance assessment, such quantitative estimates provide a basis to support remedial and/or regulatory decisions regarding the continued need for vadose zone volatile organic compound remediation or technical justification for SVE termination, which can significantly reduce the cost to complete for a site.

Design and Analysis of Breath Collection System for Rapid Analysis of Breath Condensate

Introduction Breath analysis has attracted a lot of interest as a less-invasive method for health diagnostics and the detection of biomarker for various illnesses. A number of research studies have examined miniaturization of the sensing devices to achieve portability and lower the cost so that point of care application can be considered [1]. However, one major issue in breath analysis is that the breath sample needs some preconditioning before sensing can be carried out. Specifically, the water droplets and water vapor fraction should be separated from the volatile organic compounds. [2]. For that reason, a miniaturized water vapor and suspended droplet extraction device would be useful for a portable breath analysis system. The device must be able to provide a dry breath sample gas at the outlet as well as collecting the extracted condensate which contains biomarkers for diseases [3]. In this paper, a serpentine-type mini channel [4] is proposed as miniaturized water vapor extraction device. An analytical model is developed to obtain the height, width, and length values of the channel that is sufficient to remove moisture content in exhaled breath as it is cooled by using thermoelectric element. Method An analytical mathematical model is developed to calculate the optimized channel dimensions with design objectives of maximum condensing water vapor, minimum overall space, and minimum pressure drop. Based on calculation, the geometrical dimensions of 3 x 0.5 x 30 mm (width x height x length) are selected. The mini channel is made of serpentine-patterned aluminum sheet and a lid made of PMMA (Figure 1a). The aluminum sheet and PMMA are hermetically bonded by using cyanoacrylate adhesive. To verify the calculated outlet temperature and water vapor concentration of breath flow as well as liquid collection rate of the device, an experiment using artificial breath flow is conducted with the setup as shown schematically in figure 1a. An artificial breath flow is generated by pumping ambient air at 0.06 L/s using a mini diaphragm pump (SparkFun Electronics RP 10398) through a water bubbler heated with a strip heater (HT640). It generates a humid air with RH 100% and temperature varied from 27-38oC in the mini channel inlet. The mini channel is cooled by a thermoelectric (Peltier) element (CP85438 40x40 mm) to a cold side temperature of 16oC. At the inlet and outlet side of the mini channel, a DHT22 sensor is attached to measure temperature and humidity value at both ends. Results and Conclusions Figure 1b and 1c shows the plot of temperature and water molecule concentration at the inlet and outlet side of the channel. Based on the result, the calculated temperature and water vapor concentration values have a good agreement with the measured values with margin of error ±10%. With the thermoelectric element cooled up to 16 oC, a moisture removal efficiency of 69.5% is achieved. In this case, the steady state liquid collection rate is 2 x 10-3 ml/s. It produces gas phase breath sample with RH 58% at room temperature (25 oC). Using the same model, calculation is further carried to obtain that theoretical removal efficiency of 90% can be achieved by cooling down the Peltier element to 0oC. In this case, the steady state liquid collection rate is 2.4 x 10-3 ml/s, and the device will produce gas phase breath sample with RH 18% at room temperature.

Box Experiment Study of Thermally Enhanced SVE for Benzene.

In order to describe the changes of soil temperature field, air flow field and remediation situation with time during the process of thermally enhanced SVE (soil vapor extraction), a remediation experiment of benzene contaminated soil with single extraction pipe was carried out in a box device. The results showed that the whole temperature of the system was raised to 80 °C in 4 h. 43% of benzene were removed in the first 2% of the extraction time. After 24 h, the repair efficiency was close to 100%. The device can efficiently remove benzene from soil. By continuously monitoring the parameters in the operation process of the system, the spatial distribution of temperature and soil gas pollutant concentration with time was plotted. It showed the benzene concentration distribution in the soil gas was more consistent with the temperature distribution before the start of ventilation, and the concentration of benzene in the soil gas dropped rapidly after ventilation, while the temperature distribution was almost unaffected. In the treatment of soil with a benzene content of 17.8 mg∙kg−1, when the soil gas benzene concentration is the highest at 180 min, the peak value is 11,200 mg∙m−3, and the average concentration is 7629.4 mg∙m−3.

Thermal non-equilibrium in spray flash with vacuum vapor extraction

The spray flash is the core procedure in spray flash desalination, which is an intense phase-changing process with great potential in efficient low-temperature evaporation and high evaporation capacity. In a vacuuming spray flash with active vapor extraction and absence of external heating, the latent heat is self-supplied by the evaporating droplets, resulting in a temperature difference between continued cooling droplets and the hotter yield vapor along with the spray transport. Understanding such thermal non-equilibrium between vapor and droplets is curial and beneficial to desalination industry for improving evaporation efficiency and energy recovery. This paper aims to establish an integrated experiment-modeling-CFD methodology to study the spray flash and its thermal non-equilibrium under active vacuuming. In this work, various effects on the spray flash, including the polydispersion of atomized sprays, the heat conduction coupled evaporation of droplets, as well as the non-uniformity in vapor transport, are discussed. The proposed methodology is based on a Lagrangian-Eulerian modeling approach, which is calculated by a simplified process model and numerically solved via computational fluid dynamics (CFD) of ANSYS FLUENT with user-defined functions (UDF), respectively. Two experimental systems are developed: One is for calibrating droplets' velocity and size distribution; the other is a lab-scale vacuum spray flash system for measuring the evaporative characteristics. A dimensionless corrective method for the lumped heat capacity model is proposed in order to improve the accuracy of discrete phase modeling in CFD simulation. Theoretical predictions by the process model and CFD simulation meet good agreements with experiments. Case studies of the operation parametric effects on flash characteristics are demonstrated via both modeling predictions and experimental measurements. The results indicate that the temperature difference between the extracted vapor and the discharged water can be effectively generated during the process. Such a gap is positively impacted by the operating vacuum level and the initial spray temperature. The reported thermal non-equilibrium phenomenon indicates highly efficient evaporation of spray flash in an active-vacuuming environment, and such potential could be further utilized in desalination.

Pore-Scale Modelling of Solvent-Based Recovery Processes

A need for a reduction in energy intensity and greenhouse gas emissions of bitumen and heavy oil recovery processes has led to the invention of several methods where mass-transfer-based recovery processes in terms of cold or heated solvent injection are used to reduce bitumen viscosity rather than steam injection. Despite the extensive numerical and experimental investigations, the field results are not always aligned to what is predicted unless several history matches are done. These discrepancies can be explained by investigating the mechanisms involved in mass transfer and corresponding viscosity reduction at the pore level. A two-phase multicomponent pore-scale simulator is developed to be used for realistic porous media simulation. The simulator developed predicts the chamber front velocity and chamber propagation in agreement with 2D experimental data in the literature. The simulator is specifically used for vapor extraction (VAPEX) modelling in a 2D porous medium. It was found that the solvent cannot reach its equilibrium value everywhere in the oleic phase confirming the non-equilibrium phase behavior in VAPEX. The equilibrium assumption is found to be invalid for VAPEX processes even at a small scale. The model developed can be used for further investigation of mass transfer-based processes in porous media.

Effect of water-level fluctuation on the removal of benzene from soil by SVE

In areas that experience frequent fluctuations in groundwater levels, changes in soil void space caused by fluctuations in groundwater levels directly affect the efficacy of soil vapor extraction (SVE). Most studies of SVE technology of oil-contaminated soil remediation do not consider fluctuations in groundwater levels. Here, we constructed an experimental device to study the removal of benzene by SVE under fluctuating groundwater levels. Key parameters affecting the remediation effect of SVE, such as the extraction flow, extraction time, extraction method, initial soil moisture content and initial pollutant content, were studied to characterize their effects on the efficacy of benzene removal by SVE under stable and fluctuating groundwater levels. The removal rate of benzene by SVE was approximately 10% higher under fluctuating water levels than under stable water levels. Extraction flow can directly change the removal rate under fluctuating groundwater levels. Under fluctuating groundwater levels, the removal effect of SVE on benzene in different soils of the “stabilization zone - fluctuation zone - saturation zone” also significantly differed; specifically, the removal effect of SVE in the fluctuation zone and stabilization zone was superior to that in the saturation zone. Given the fluctuation in groundwater levels at the study site, the remediation.

WITHDRAWN: Simulation study and economic analysis of steam-assisted vapor extraction for heavy oil recovery in thin reservoirs

This article has been withdrawn: please see Elsevier Policy on Article Withdrawal http://www.elsevier.com/locate/withdrawalpolicy . This article has been withdrawn at the request of the editor and publisher. The publisher regrets that an error occurred which led to the premature publication of this paper. This error bears no reflection on the article or its authors. The publisher apologizes to the authors and the readers for this unfortunate error.

Research progress in the enhancement technology of soil vapor extraction of volatile petroleum hydrocarbon pollutants.

At present, volatile petroleum hydrocarbon pollution in soil is still widespread and serious in China. Among the treatment methods for contaminated soil, soil vapor extraction technology is widely used in in situ remediation of volatile petroleum hydrocarbon-contaminated soil due to its simple equipment, easy installation, convenient operation and high remediation efficiency. In this paper, the present situation of volatile organic compounds (VOCs) contaminated by petroleum hydrocarbons is reviewed. The effects of soil permeability, pollutant characteristics, soil moisture content, ambient temperature, and gas extraction flow rate on soil vapor extraction are discussed. Moreover, five different types of soil vapor extraction enhancement technologies, characteristics, research and applications are introduced, and the development prospect of soil remediation industry is presented.

Time-lapse electrical resistivity tomography mapping of DNAPL remediation at a STAR field site

Time-lapse Electrical Resistivity Tomography (ERT), a surface geophysical technique, was applied for the first time to directly map in-situ DNAPL destruction at a field site. Time-lapse ERT was used to map remediation of coal tar at a site undergoing full-scale application of Self-sustaining Treatment for Active Remediation (STAR). STAR is an in situ thermal treatment based on smoldering combustion that destroys dense non-aqueous phase liquids (DNAPLs) while generating heat, water, and combustion gases. ERT was used to provide spatially and temporally resolved data to complement the conventional discrete and overview data normally collected during STAR operations. A STAR treatment cell targeting coal tar at 2.4 mbgs and below the water table was monitored with 2D surface resistivity surveys conducted before, during and after treatment. Two 36-electrode lines, each 21 m long, were employed. Relative resistivity ratio maps and discrete (nodal) resistivity signal analysis both proved useful in numerous ways. For example, ERT was demonstrated to map likely treated coal-tar zones. This was accomplished by identifying a characteristic, local, temporal resistivity signal pattern corresponding to STAR-induced subsurface temperature and liquid saturation changes. In addition, ERT identified the subsurface areas (i) effectively captured by the vapor extraction system, (ii) possibly missed by the vapor capture system where gases built-up, and (iii) where and when re-infiltration of groundwater occurred post-treatment. In addition, the overall average subsurface resistivity provided a useful metric to correlate with traditional site measures of treatment such as the cumulative amount of DNAPL mass treated. This study provides a successful proof of concept however the maximum potential of the approach is expected with a full 3D survey. Overall, the resistivity surveys provided valuable continuous mapping of the subsurface, including the locations of DNAPL mass destruction, and showed that ERT is promising for supporting evaluations of in situ remediation programs.